U.S. patent application number 09/910464 was filed with the patent office on 2002-03-21 for method and apparatus for reading and writing a multilevel signal from an optical disc.
Invention is credited to Fan, John L., Lee, David C., Ling, Yi, Lo, Yung-Cheng, Martin, Richard L., McLaughlin, Steven W., Mcpheters, Laura L., Powelson, Judith C., Spielman, Steven R., Warland, David K., Wong, Terrence L., Zingman, Jonathan A..
Application Number | 20020034138 09/910464 |
Document ID | / |
Family ID | 22961799 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020034138 |
Kind Code |
A1 |
Wong, Terrence L. ; et
al. |
March 21, 2002 |
Method and apparatus for reading and writing a multilevel signal
from an optical disc
Abstract
A system and method are disclosed for reading a multilevel
signal from an optical disc. The method includes reading a raw
analog data signal from a disc using an optical detector and
adjusting the amplitude of the raw analog data signal. A timing
signal is recovered from the amplitude adjusted analog data signal
and correction is made for amplitude modulation of the raw analog
data signal by processing the raw analog data signal and the timing
signal.
Inventors: |
Wong, Terrence L.; (San
Francisco, CA) ; Fan, John L.; (Palo Alto, CA)
; Lee, David C.; (The Peak, HK) ; Ling, Yi;
(Foster City, CA) ; Lo, Yung-Cheng; (San Leandro,
CA) ; McLaughlin, Steven W.; (Decatur, GA) ;
Mcpheters, Laura L.; (Austin, TX) ; Martin, Richard
L.; (Berkeley, CA) ; Powelson, Judith C.;
(Alameda, CA) ; Spielman, Steven R.; (Berkeley,
CA) ; Warland, David K.; (Davis, CA) ;
Zingman, Jonathan A.; (Oakland, CA) |
Correspondence
Address: |
RITTER VAN PELT & YI, L.L.P.
4906 EL CAMINO REAL
SUITE 205
LOS ALTOS
CA
94022
|
Family ID: |
22961799 |
Appl. No.: |
09/910464 |
Filed: |
July 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09910464 |
Jul 19, 2001 |
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09253808 |
Feb 18, 1999 |
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6275458 |
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Current U.S.
Class: |
369/47.35 ;
G9B/20.01; G9B/7.01; G9B/7.018; G9B/7.034; G9B/7.038 |
Current CPC
Class: |
G11B 7/00745 20130101;
G11B 27/24 20130101; G11B 2020/1275 20130101; G11B 20/1403
20130101; G11B 20/1833 20130101; G11B 2220/2537 20130101; G11B
27/3027 20130101; G11B 7/005 20130101; G11B 20/10009 20130101; G11B
2020/1268 20130101; H03M 13/00 20130101; G11B 7/0045 20130101; G11B
20/1252 20130101; G11B 2020/1222 20130101; G11B 2020/1287 20130101;
G11B 2220/20 20130101; G11B 7/013 20130101; H03M 13/23 20130101;
G11B 27/19 20130101 |
Class at
Publication: |
369/47.35 |
International
Class: |
G11B 007/005 |
Claims
What is claimed is:
1. A method of reading a multilevel signal from an optical disc
comprising: reading a raw analog data signal from a disc using an
optical detector; preliminarily correcting for amplitude modulation
in the analog data signal to obtain a preliminarily corrected
analog data signal; recovering a timing signal from the
preliminarily corrected analog data signal; and correcting for
amplitude modulation of the raw analog data signal by processing
the raw analog data signal and the timing signal.
2. A method of reading a multilevel signal from an optical disc as
recited in claim .backslash.* MERGEFORMAT 1 wherein correcting for
amplitude modulation of the raw analog data signal includes
evaluating the value of the raw analog data signal at times
determined to correspond to gain control fields.
3. A method of reading a multilevel signal from an optical disc as
recited in claim 1 wherein the times determined to correspond to
gain control fields are determined using the recovered timing
signal.
4. A method of reading a multilevel signal from an optical disc as
recited in claim 3 .backslash.* MERGEFORMAT wherein correcting for
amplitude modulation of the raw analog data signal includes
detecting an envelope of the raw analog data signal and normalizing
the raw data signal.
5. A method of reading a multilevel signal from an optical disc as
recited in claim .backslash.* MERGEFORMAT 1 wherein using the
timing signal to further correct for amplitude modulation in the
amplitude adjusted analog data signal further includes normalizing
the adjusted analog data signal based on the strength of the signal
read at a gain control field.
6. A method of reading a multilevel signal from an optical disc as
recited in claim 5 .backslash.* MERGEFORMAT wherein normalizing the
adjusted analog data signal based on the strength of the signal
read at a gain control field includes detecting the strength of the
signal at a point near the center of the gain control field and
wherein the center of the gain control field is found using the
timing signal.
7. A method of reading a multilevel signal from an optical disc
comprising: reading a raw analog data signal from a disc using an
optical detector; recovering a timing signal from the raw analog
data signal; converting the analog signal to a digital data signal
using an A/D converter; and correcting for amplitude modulation of
the raw analog data signal by processing the digital data signal to
obtain an amplitude adjusted digital data signal.
8. A method of reading a multilevel signal from an optical disc as
recited in claim 7 .backslash.* MERGEFORMAT wherein the timing
signal is input to the A/D converter.
9. A method of reading a multilevel signal from an optical disc as
recited in claim 7 .backslash.* MERGEFORMAT wherein the amplitude
of the raw analog data signal is adjusted before the timing signal
is recovered.
10. A method of reading a multilevel signal from an optical disc
comprising: reading a raw analog data signal from a disc using an
optical detector; correcting for amplitude modulation of the raw
analog data signal by processing the raw analog data signal to
obtain an amplitude adjusted analog data signal; recovering a
timing signal from the amplitude adjusted analog data signal; and
using the timing signal to further correct for amplitude modulation
in the amplitude adjusted analog data signal.
11. A method of reading a multilevel signal from an optical disc
comprising: reading a raw analog data signal from a disc using an
optical detector; correcting for amplitude modulation of the raw
analog data signal by processing the raw analog data signal to
obtain an amplitude adjusted analog data signal; recovering a
timing signal from the amplitude adjusted analog data signal;
converting the amplitude adjusted analog data signal to a digital
data signal; processing the digital data signal with a fractionally
spaced equalizer to obtain an equalized data signal.
12. A method of reading a multilevel signal from an optical disc as
recited in claim 11 wherein the fractionally spaced equalizer is
adaptive.
13. A method of reading a multilevel signal from an optical disc as
recited in claim 11 wherein the fractionally spaced equalizer is
has taps and wherein the fractionally spaced equalizer trains on a
training sequence to set the taps.
14. A method of reading a multilevel signal from an optical disc as
recited in claim 11 further including processing the equalized data
signal using a Viterbi detector to output a recovered data
sequence.
15. A method of reading a multilevel signal from an optical disc as
recited in claim 14 further wherein the Viterbi detector removes
the effect of non data marks from the equalized data signal
16. A method of reading a multilevel signal from an optical disc as
recited in claim 11 .backslash.* MERGEFORMAT wherein the equalized
data signal is equalized to a target of 1+D.
17. A method of reading a multilevel signal from an optical disc as
recited in claim 11 .backslash.* MERGEFORMAT wherein the equalized
data signal is equalized to remove the effects of intersymbol
interference. .backslash.* MERGEFORMAT
18. A method of reading a multilevel signal from an optical disc as
recited in claim .backslash.* MERGEFORMAT 11 further including
decoding the recovered data sequence and detecting errors in the
recovered data sequence.
19. A method of reading a multilevel signal from an optical disc as
recited in claim 11 .backslash.* MERGEFORMAT further including
decoding the recovered data sequence and correcting errors in the
recovered data sequence.
20. A method of reading a signal from an optical disc comprising:
reading a raw analog data signal from a disc using an optical
detector, the raw analog data signal including an alignment
sequence wherein the alignment sequence is chosen such that the
autocorrelation of the alignment sequence has a substantially high
value at a single alignment point; converting the raw analog data
signal to a digital data signal; and cross correlating the digital
data signal with a stored digital version of the alignment
sequence; whereby the start of a data sequence can be
determined.
21. A method of reading a signal from an optical disc as recited in
claim 20 .backslash.* MERGEFORMAT wherein the signal is a
multilevel signal.
22. A multilevel pattern of marks written to an optical disc
including: a preamble including: a timing acquisition sequence of
fields; an alignment sequence; a calibration sequence of marks; and
an equalizer training section; and a data block.
23. A multilevel pattern of marks written to an optical disc as
recited in claim 22 wherein the preamble further includes a data
block address.
24. A multilevel pattern of marks written to an optical disc
including: a data block; and a postamble including: a dc
compensation region that adjusts the overall dc level of the data
block and any associated preamble and postamble to substantially
zero;
25. A multilevel pattern of marks written to an optical disc as
recited in claim 24 including one or more trellis code clean-up
marks that returns the trellis encoded data to a known state.
26. A multilevel pattern of marks written to an optical disc
organized into an ECC block including: modulation encoded marks
that include encoded data marks; and physical format marks that
include: periodic ECC data synch fields, periodic timing fields,
periodic AGC fields, and periodic DC control fields.
27. A multilevel pattern of marks written to an optical disc
organized into an ECC block as recited in claim 26 wherein the
modulation encoded marks further include an encoded address
section.
28. An ECC block as recited in claim 27 further including one or
more trellis code clean-up mark that return the trellis code to a
known state.
29. A method of recording data on an optical disc including:
defining a desired data sequence; deriving a write signal from the
desired data sequence using a write strategy; recording the optical
disc using the write signal; reading the optical disc to obtain a
recovered sequence; comparing the recovered sequence to the desired
data sequence; adjusting the write strategy based on the comparison
of the recovered sequence to the desired data sequence so that the
recovered sequence tends to converge toward the desired data
sequence.
30. A method of recording data on an optical disc including:
defining a desired data sequence; deriving a write signal from the
desired data sequence using a write strategy; recording the optical
disc using the write signal; reading the optical disc to obtain a
recovered sequence; linearly filtering the desired data sequence;
comparing the recovered sequence to the linearly filtered desired
data sequence; and adjusting the write strategy based on the
comparison of the recovered sequence to the linearly filtered
desired data sequence so that the recovered sequence tends to
converge toward the linearly filtered desired data sequence.
31. A multilevel pattern of marks written to an optical disc
including: data marks that include more than two levels of data;
timing fields occurring periodically between the data marks; and
automatic gain control fields occurring periodically between the
data marks wherein the automatic gain control fields correspond to
a specific level of data.
32. A multilevel pattern of marks written to an optical disc as
recited in claim .backslash.* MERGEFORMAT 31 wherein the specific
level of data is a maximum level of data or a minimum level of
data.
33. A multilevel pattern of marks written to an optical disc as
recited in claim 31 .backslash.* MERGEFORMAT further including dc
control fields that maintain a substantially constant dc level when
the optical disc is read.
34. A multilevel pattern of marks written to an optical disc as
recited in claim 31 .backslash.* MERGEFORMAT wherein the timing
fields include data written at a highest level for a consecutive
sequence of marks followed by data written at a lowest level for a
consecutive sequence of marks.
35. A multilevel pattern of marks written to an optical disc as
recited in claim 31 .backslash.* MERGEFORMAT wherein the automatic
gain control fields occur periodically among certain timing
fields.
36. A multilevel pattern of marks written to an optical disc as
recited in claim 31 .backslash.* MERGEFORMAT further including DC
control fields corresponding to DC control field output signal
levels wherein the output signal levels corresponding to the DC
control fields compensate for DC variation in an output signal.
37. A multilevel pattern of marks written to an optical disc as
recited in claim 31 .backslash.* MERGEFORMAT further including an
alignment sequence of marks.
38. A multilevel pattern of marks written to an optical disc as
recited in claim 31 wherein the alignment sequence of marks is a
pseudo random sequence that has a response such that if correlated
with itself that the correlation gives a high value only at one
location.
39. A method of writing a multilevel signal to an optical disc
comprising: encoding data by mapping the data onto a plurality of
levels including more than two levels; adding synchronization
fields to the encoded data; determining a DC level of the encoded
data; adding DC control fields to keep the DC level of the encoded
data substantially constant.
40. A method of writing a multilevel signal to an optical disc as
recited in claim 39 .backslash.* MERGEFORMAT further including
adding automatic gain control fields to the encoded data to provide
a predetermined read signal pattern for the purpose of adjusting
gain of a read signal.
41. A method of writing a multilevel signal to an optical disc
including: determining a raw data sequence having raw data sequence
elements; and encoding the raw data sequence using a convolutional
code to obtain a correlated data sequence wherein the correlated
data sequence has correlated data sequence elements that are a
function of more than one element of the raw data sequence; and
writing the correlated data sequence to the optical disc.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a method and
apparatus for reading multilevel signals from an optical disc and
writing multilevel signals to an optical disc. The invention
relates to methods and apparatuses for processing signals that are
eventually written to and read from an optical disc. These signals
produce marks on the optical disc that may vary in both
reflectivity and length. The system disclosed provides a method of
encoding and decoding the data, correcting for errors,
synchronizing the data, controlling the DC content, establishing
and recovering a clock signal, establishing and recovering the
envelope of the signal, and compensating for signal distortion.
[0003] 2. Relationship to the Art
[0004] In order to increase the capacity and speed of optical data
storage systems, multilevel optical recording systems have been
developed. It should be noted that in this specification, the term
multilevel is used to indicate greater than 2 levels. In a
traditional optical recording system, reflectivity of the recording
media is modulated between two states. The density of data recorded
on an optical recording medium may be increased by modulating the
reflectivity of the optical recording medium into more than two
states.
[0005] One type of optical recording medium that appears to be
particularly suitable for multilevel signal modulation is phase
change optical material. When a phase change material is heated by
a writing laser, the reflectivity of the phase change material may
be changed. The change in reflectivity may be controlled by
adjusting the amount of heating of the material and the rate at
which the material cools. This process is described further in
"Laser-induced crystallization phenomena in GeTe-based alloys. I.
Characterization on nucleation and growth" (J. Appl. Phys. 78 (8),
Oct. 15, 1995. p. 4906) by J. H. Coombs, et. al. (hereinafter
"Coombs").
[0006] After a phase change optical disc has been written, the
intensity of a beam of light reflected from the disc is measured so
that the multilevel data written to the disc may be recovered. U.S.
Pat. No. 5,144,615 entitled APPARATUS AND METHOD FOR RECORDING AND
REPRODUCING MULTILEVEL INFORMATION issued to Kobayashi (hereinafter
"Kobayashi") discloses a system for recovering multilevel data from
such an optical disc. FIG. 1 is a block diagram illustrating the
system disclosed in Kobayashi for recovering such data. Analog data
read from a detector is input from a mark length detecting circuit
101 and a reflectivity detecting circuit 102. The outputs of these
circuits are sent to an analog-to-digital (A/D) converter 103. The
A/D converter 103 includes an n-value circuit which determines the
value that the signal corresponds to by comparing the signal to
predetermined reference voltages. Subsequently, the n-value signal
is converted into a binary signal by binary circuit 405.
[0007] While this system discloses the concept of reading a
multilevel signal and converting it into a digital signal in a
basic sense, no method is disclosed of handling various
imperfections in optically read multilevel signals that in fact
tend to occur. For example, it is not clear how a clock is
recovered for the purpose of precisely detecting mark lengths and
no method is disclosed for handling problems that tend to occur in
real systems such as amplitude modulation and DC offset of the
optically detected signal and noise.
[0008] In a conventional two level optical data storage system,
information is stored in the lengths of the marks and the spaces
between them. So long as the edge of a mark can be detected with
enough precision to distinguish between marks that differ in length
by a minimum allowed amount, the system can operate reliably. This
edge transition between one reflectivity state and another can be
detected by setting a threshold value and determining the time when
the signal crosses the threshold. Slow amplitude variations that
might interfere with this edge detection are removed by AC coupling
the photodetector signal before the threshold detection circuit.
Mark and space lengths are measured by counting how many clock
periods are between the edge transitions. The reader clock periods
are synchronized to the mark/space edges, thus ensuring that there
are an integral number of clock periods in each mark/space.
[0009] In contrast, in a multilevel recording system, it is the
amplitude of the signal that carries information. The reader must
interpret the data signal to determine the amplitude of the signal
at certain times. Therefore, the reader clock must be synchronized
to the data stream to ensure that the reader is interpreting the
signal at the proper time. Because of the blurring effect of the
optics in a reader, the transitions between the different levels do
not create sharp edges. It is therefore difficult to synchronize
the reader clock to the data stream. A method of precisely aligning
a read data stream is needed. Further, a multilevel system is more
sensitive to fluctuations in the overall envelope of the data
signal. AC coupling alone is not adequate to enable a sufficiently
precise determination of the different amplitude signals. Another
problem encountered in a multilevel optical disc system is DC
compensation.
[0010] In order for a multilevel optical read system to reliably
record and recover data, a method of handling these sources of
error in reading an optical signal is needed.
SUMMARY OF THE INVENTION
[0011] Accordingly, a system for writing and reading multilevel
marks on an optical disc is disclosed. The system includes an error
correction encoding and decoding system, modulation and
demodulation system, DC control system, amplitude correction
circuit, a clock recovery circuit, a write strategy system, a
system to focus and track the laser spot on the surface of the
disc, a system to rotate the disc, and an interface to a computer
system. It should be appreciated that the present invention can be
implemented in numerous ways, including as a process, an apparatus,
a system, a device, a method, or a computer readable medium that
includes certain types of marks that enable reliable data storage
and recovery. Several inventive embodiments of the present
invention are described below.
[0012] In one embodiment, method is disclosed for reading a
multilevel signal from an optical disc. The method includes reading
a raw analog data signal from a disc using an optical detector and
adjusting the amplitude of the raw analog data signal. A timing
signal is recovered from the amplitude adjusted analog data signal
and correction is made for amplitude modulation of the raw analog
data signal by processing the raw analog data signal and the timing
signal.
[0013] In another embodiment, a method of reading a multilevel
signal from an optical disc includes reading a raw analog data
signal from a disc using an optical detector and recovering a
timing signal from the raw analog data signal. The analog signal is
converted to a digital data signal using an A/D converter.
Amplitude modulation of the raw analog data signal is corrected by
processing the digital data signal to obtain an amplitude adjusted
digital data signal.
[0014] In another embodiment, a method of reading a multilevel
signal from an optical disc includes reading a raw analog data
signal from a disc using an optical detector and correcting for
amplitude modulation of the raw analog data signal by processing
the raw analog data signal to obtain an amplitude adjusted analog
data signal. A timing signal is recovered from the amplitude
adjusted analog data signal and the timing signal is used to
further correct for amplitude modulation in the amplitude adjusted
analog data signal.
[0015] In another embodiment, A method of reading a multilevel
signal from an optical disc includes reading a raw analog data
signal from a disc using an optical detector and correcting for
amplitude modulation of the raw analog data signal by processing
the raw analog data signal to obtain an amplitude adjusted analog
data signal. A timing signal is recovered from the amplitude
adjusted analog data signal. The amplitude adjusted analog data
signal is converted to a digital data signal. The digital data
signal is processed with a fractionally spaced equalizer to obtain
an equalized data signal.
[0016] In another embodiment, a method of reading a signal from an
optical disc includes reading a raw analog data signal from a disc
using an optical detector. The raw analog data signal includes an
alignment sequence. The alignment sequence is chosen such that the
autocorrelation of the alignment sequence has a substantially high
value at a single alignment point. The raw analog data signal is
converted to a digital data signal. The digital data signal is
cross correlated with a stored digital version of the alignment
sequence so that the start of a data sequence can be
determined.
[0017] In another embodiment, a multilevel pattern of marks written
to an optical disc includes a preamble and a data block. The
preamble includes a timing acquisition sequence of fields, an
alignment sequence, a calibration sequence of marks, and an
equalizer training section.
[0018] In another embodiment, A multilevel pattern of marks written
to an optical disc organized into an ECC block includes modulation
encoded marks and physical format marks. The modulation encoded
marks include an encoded address section and encoded data marks.
The physical format block marks include periodic ECC data synch
fields, periodic timing fields, periodic AGC fields, and periodic
DC control fields.
[0019] In another embodiment, a method of recording data on an
optical disc includes defining a desired data sequence and deriving
a write signal from the desired data sequence using a write
strategy. The optical disc is recorded using the write signal and
the optical disc is read to obtain a recovered sequence. The
recovered sequence is compared to the desired data sequence and the
write strategy is adjusted based on the comparison of the recovered
sequence to the desired data sequence so that the recovered
sequence tends to converge toward the desired data sequence.
[0020] In another embodiment, a method of recording data on an
optical disc includes defining a desired data sequence and deriving
a write signal from the desired data sequence using a write
strategy. The optical disc is recorded using the write signal and
the optical disc is read to obtain a recovered sequence. The
desired data sequence is linearly filtered. The recovered sequence
is compared to the linearly filtered desired data sequence and the
write strategy is adjusted based on the comparison of the recovered
sequence to the linearly filtered desired data sequence so that the
recovered sequence tends to converge toward the linearly filtered
desired data sequence.
[0021] In another embodiment, a multilevel pattern of marks written
to an optical disc includes data marks that include more than two
levels of data, timing fields occurring periodically between the
data marks, and automatic gain control fields occurring
periodically between the data marks wherein the automatic gain
control fields correspond to a specific level of data.
[0022] In another embodiment, a method of writing a multilevel
signal to an optical disc includes encoding data by mapping the
data onto a plurality of levels including more than two levels,
adding synchronization fields to the encoded data, determining a DC
level of the encoded data, and adding DC control fields to keep the
DC level of the encoded data substantially constant.
[0023] In another embodiment, a method of writing a multilevel
signal to an optical disc includes determining a raw data sequence
having raw data sequence elements and encoding the raw data
sequence using a convolutional code to obtain a correlated data
sequence. The correlated data sequence has correlated data sequence
elements that are a function of more than one element of the raw
data sequence. The correlated data sequence is written to the
optical disc.
[0024] These and other features and advantages of the present
invention will be presented in more detail in the following
specification of the invention and the accompanying figures which
illustrate by way of example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The present invention will be readily understood by the
following detailed description in conjunction with the accompanying
drawings, wherein like reference numerals designate like structural
elements, and in which:
[0026] FIG. 1 is a block diagram illustrating the system disclosed
in Kobayashi for recovering such data.
[0027] FIG. 2 is a block diagram of a system for reading a
multilevel signal from an optical disc.
[0028] FIG. 3A is a block diagram illustrating the components of a
signal processing system.
[0029] FIG. 3B is a block diagram illustrating an alternative
desnaking architecture.
[0030] FIG. 4 illustrates how the preliminary desnaker processes an
amplitude modulated signal.
[0031] FIG. 5A is a block diagram showing a data block format used
in one embodiment.
[0032] FIG. 5B illustrates a preamble sequence.
[0033] FIG. 5C illustrates an ECC block.
[0034] FIG. 5D illustrates a postamble sequence.
[0035] FIG. 6 is a block diagram illustrating a system for writing
multilevel marks to a disc.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] Reference will now be made in detail to the preferred
embodiment of the invention. An example of the preferred embodiment
is illustrated in the accompanying drawings. While the invention
will be described in conjunction with that preferred embodiment, it
will be understood that it is not intended to limit the invention
to one preferred embodiment. On the contrary, it is intended to
cover alternatives, modifications, and equivalents as may be
included within the spirit and scope of the invention as defined by
the appended claims. In the following description, numerous
specific details are set forth in order to provide a thorough
understanding of the present invention. The present invention may
be practiced without some or all of these specific details. In
other instances, well known process operations have not been
described in detail in order not to unnecessarily obscure the
present invention.
[0037] FIG. 2 is a block diagram of a system for reading a
multilevel signal from an optical disc. A disc 200 is rotated by a
motor 202. An optical head 204 includes a laser that illuminates a
storage location on the disc and a multi-element photodetector that
detects the reflected light from the location. The optical disc
contains a multilevel modulated signal. The multilevel signal may
be written to the optical disc in different embodiments using
various techniques for changing the reflectivity of the disc
including varying pit depth, varying the exposure of a dye, or
changing the phase of an optical phase change material. In the
example illustrated below, an optical phase change material is
used. The photodetector converts the light into an electrical
signal which is converted from a current to a voltage and amplified
by an pre-amplifier 210. The output of the pre-amplifier 210 is fed
to a servo error-signal calculator 212 that calculates combinations
of the signal for the focus, tracking, and data interpretation
systems. The data signal contains the multilevel information and in
prior art systems such as the one described above, the amplified
data is input into an A/D converter for the purpose of digitizing
that information. An improved system is disclosed herein that
includes various signal processing stages between the amplifier and
the A/D converter as well as after the A/D converter.
[0038] The signal from servo error-signal calculator 212 is input
to a signal processing system 214 before it is input into an analog
to digital (A/D) converter 218. The signal processing system 214 is
further described in FIG. 3. In addition to the processed data
signal, a clock signal is generated by signal processing system 214
and the clock signal is also provided to A/D converter 218. The
digitized output signal from the A/D converter 220 is input into an
adaptive fractionally spaced equalizer (FSE) 230. This adaptive FSE
shapes the frequency response of the data signal so that the data
channel collectively gives a specific amount of intersymbol
interference. In addition, the adaptive nature of the FSE allows
the system to accommodate interchange of media and players and
inhomogeneities in the media. The output of the adaptive FSE is
input into a Viterbi decoder 234 that includes a mark handler.
[0039] The mark handler is used to separate special timing and gain
control fields, DC control fields, and other fields that do not
carry data from storage locations on the disc that actually
represent data. Timing and gain control fields are described below.
DC control fields are periodically written to the disc to adjust
the average signal level written to the disc so that low frequency
content of the signal is suppressed. These DC control fields do not
carry information and are used simply to avoid having a DC bias on
the read signal.
[0040] The Viterbi decoder recovers the sequence of data encoded on
the disc. The output of the Viterbi decoder is input to an error
correction code decoder 236 and the output data from the error
correction code decoder is made available to the system reading
data from the optical disc.
[0041] FIG. 3A is a block diagram illustrating a desnaker that may
be included in the components of signal processing system 214. An
analog signal is fed into a preliminary desnaker 302 and also to a
clocked desnaker 308. The purpose of the desnaker is to remove the
effect of amplitude modulation on the signal. The desnaker removes
modulation in the envelope of the signal caused by variations in
the characteristics of the optical disc or mechanical variations.
For example, disc warp may cause amplitude modulation of the read
signal separate from the recorded data. Also, the amount of phase
change optical material deposited may vary, or there may be
variations in the index of refraction or the thickness of the
polycarbonate material covering the surface of the disc. These
variations in the envelope of the data signal cause data read
errors as well as timing errors.
[0042] FIG. 4 illustrates how the preliminary desnaker processes an
amplitude modulated signal. The desired signal 402 does not have
variation in its envelope. A raw signal 404 typically has some
variation in its envelope. This varying envelope resembles the
shape of a snake, hence the term "desnaking". In one embodiment,
the envelope variation is removed by using top and bottom envelope
detector circuits. The output of a top envelope detector circuit
applied to amplitude modulated signal 404 is shown as signal 406.
Signal 406 follows the peaks of the raw signal. Similarly, the
output of a bottom envelope detector circuit follows the minimums
of the raw signal.
[0043] The top and bottom envelope detection circuit outputs are
used to normalize the amplitude. The amplitude modulation is
removed by subtracting off the bottom signal from the data signal
and then dividing by top minus bottom. In other words, the offset
is subtracted off of the data signal, and then the amplitude is
normalized. The design of peak detectors is described in The Art of
Electronics by Horowitz and Hill which is herein incorporated by
reference.
[0044] The output of preliminary desnaker 302 is input into a
timing recovery system 306 for the acquisition of timing fields.
Timing recovery circuit 306 acquires synchronization to timing
fields that are included in the data signal and generates a sample
clock. In one embodiment, a timing field includes a series of
storage locations, or marks, written with specific values.
Specifically, a timing field can include three storage locations
written at the highest mark signal level followed by three storage
locations written at the lowest mark signal level. In other
embodiments, timing recovery can also be achieved by synchronizing
to the data marks without specific timing fields being embedded in
the data signal.
[0045] A timing field may also be three storage locations written
at the lowest level followed by three storage locations written at
the highest level. The difference between the high to low
transition timing fields and the low to high transition timing
fields is used to distinguish the fields. In addition, in one
embodiment, every fourth timing field is five storage positions
written at the highest level followed by five storage positions
written at the lowest level or five storage positions written at
the lowest followed by five storage positions written at the
highest level. The extra long timing field that is ten storage
positions long is also used as an amplitude automatic gain control
field. The automatic gain control fields are used to more
accurately desnake or remove the amplitude modulation of the
envelope of the data signal.
[0046] Once the clock recovery circuit has locked to the timing
fields, the clocked desnaker is used to more accurately desnake the
signal. The clocked desnaker 308 receives an input from the timing
recovery circuit and also receives the raw data signal from the
optical detector. The clock desnaker 308 performs a more accurate
removal of the amplitude modulation on the signal because the
clocked desnaker uses the clock signal recovered by the clock
recovery circuit to determine a point near the center of either a
high region or low region of the automatic gain control field. Such
a point gives a reliable measure of the full amplitude response of
the media at that location, since intersymbol interference from
neighboring marks of different levels is reduced or eliminated.
Amplitude modulation of the signal is removed again by subtracting
off the offset of the signal and by multiplying the signal by a
value that is inversely proportional to the amplitude detected.
Other methods of correcting for the amplitude modulation may be
used that use the signal read from the automatic gain control
field.
[0047] Thus, the amplitude modulation is initially removed using a
preliminary desnaker and the output of the preliminary desnaker is
used to acquire a clock signal. Once the clock signal is acquired,
the clock signal is used to locate portions of the raw data signal
that include automatic gain control (AGC) fields. The signal read
at the automatic gain control field locations is used to more
precisely compensate for amplitude modulation that occurs on the
disc. The more precise compensation is performed by the clocked
desnaker, and the output of the clocked desnaker is then used by
the system.
[0048] The clocked desnaker uses the timing information obtained by
the clock recovery circuit to accurately desnake the signal based
on the signal read at the positions of the automatic gain control
fields. This arrangement of a preliminary desnaker that facilitates
the clock recovery and a second desnaker using the signal obtained
from automatic gain control fields yields particularly good
results. In other embodiments, different desnaking systems are
used.
[0049] In one embodiment, a first preliminary desnaker is used that
is similar to the preliminary desnaker described above. However,
instead of a second clocked desnaker that operates on the read
analog signal, the read analog signal is digitized and a digital
desnaker is used to precisely desnake the data. In such an
embodiment, the number of bits of resolution of the digitized read
signal exceeds the number of bits of data that are encoded in the
signal. In other embodiments, the raw analog signal is digitized
without analog desnaking and all desnaking occurs in the digital
domain.
[0050] Referring back to FIG. 3A, the signal output from the
clocked desnaker is input into an anti-aliasing filter 310 and the
output of the anti-aliasing filter 310 is input to an analog to
digital (A/D) converter 312. A clock signal from the timing
recovery system is also input to the analog to digital converter.
The clock signal is used by the A/D converter to determine when to
digitize the data signal. This digitized signal is then fed to FSE
230 shown in FIG. 2. It has been discovered that the analog
desnaking prior to digitization may significantly improve system
performance in some cases. In an alternate embodiment, the data
signal is digitized by the A/D converter with only the preliminary
desnaker or with no desnaking. This signal is then desnaked in the
digital domain using a similar algorithm based on the measurement
of the automatic gain control field signal levels.
[0051] FIG. 3B is a block diagram illustrating an alternative
desnaking architecture. A read signal 350 is input to a preliminary
desnaker 351. Timing information is derived from the output of the
preliminary desnaker by a timing recovery system 352 and the timing
information is used to generate a clock for an analog to digital
converter 354, which digitizes the read signal. The output of
analog to digital converter 354 is input to a digital desnaker 356
that processes the signal to perform desnaking in the digital
domain. Desnaking is performed digitally by analyzing recovered
signal peak values and making adjustments to the signal to correct
for detected signal amplitude variation. As in the clocked analog
desnaker, automatic gain control marks may be used. The output of
analog to digital converter 354 is input to FSE 230. In one
embodiment, the analog to digital converter is a 12 bit analog to
digital converter and the FSE derives 8 levels of data.
[0052] In one embodiment, FSE is a finite impulse response (FIR)
filter with two taps for each mark. The equalizer is designed to
shape the response of the channel to a specific equalization
target. In one embodiment, the equalization target has a 1+D target
transfer function. That is, the target output signal after passing
through the optical data channel and equalization is equal to the
input signal plus the input signal delayed by a time interval. The
fractionally spaced equalizer has two advantages over a once per
mark spaced equalizer. First, the noise characteristics are better
and second, the FSE is able to correct for timing offsets. In
addition, by training on a specific sequence at the beginning of a
data block, the FSE filter can adapt to differences in individual
recorders and players that read and write marks and also to
disc/media variations. Also, the FSE filter can adapt to changes
that occur over time as an individual player experiences wear. In
another embodiment, the FSE, possibly together with the
precompensation, undoes the intersymbol interference in the data
signal. This is referred to as a zero forcing equalizer. A FSE is
described further in Proakis, "Digital Communications," 3rd
edition, which is herein incorporated by reference, at pp. 617-620,
and also in Lee & Messerschmitt, "Digital Communication," 2nd
ed., which is herein incorporated by reference at
pp.482,484,544.
[0053] Referring back to FIG. 2, the signal output from the FSE is
interpreted by a Viterbi detector 234. The Viterbi detector
interprets the signal levels and determines the most likely
sequence of data based on a metric. It calculates this metric for
all combinations of paths and, after some time, chooses the path
that was most likely as determined by the path-metric
calculation.
[0054] The Viterbi detector includes a mark handler that removes
the effect of non data marks on the equalized signal output from
the FSE. In one embodiment, the Viterbi accomplishes this by
periodically ignoring marks in the locations where nondata marks
are known to exist.
[0055] The output signal from the Viterbi is sent to an error
correction code (ECC) decoder. The ECC decoder decodes the data and
checks for errors in the data. One such error correction code used
in one embodiment is described in U.S. patent application Ser. No.
09/083,699 entitled, Method And Apparatus For Modulation Encoding
Data For Storage On A Multi-Level Optical Recording Medium by
Welch, et. al which is herein incorporated by reference.
[0056] A system for reading multilevel marks including automatic
gain control fields and decoding data from those marks has been
described. Next, the marks themselves and a system for writing such
marks is described. A typical mark pattern generated by such a
writing system and read by the read system described above is
shown.
[0057] In one embodiment, the system disclosed is used to read and
write data marks to a disc where each of the data marks are the
same length. Constraints are not imposed that require a minimum
number of identical data marks to be written consecutively or that
define a maximum number of identical data marks that may be placed
next to each other. Neither are constraints imposed that require
the data marks to periodically return to a certain level, as with a
return to zero code. In other embodiments, run length limited codes
or return to zero codes may be used. In addition to data marks,
system marks are also included periodically in the data stream to
facilitate the operation of the modules described above. Sets of
marks that together perform a function are referred to as a field.
System marks may include timing, AGC, and DC control marks that are
periodically inserted into the data stream.
[0058] FIG. 5A is a block diagram showing a data block format used
in one embodiment. The preamble (which is described further in FIG.
5B) contains sections for clock acquisition, level calibration,
equalizer training, alignment, block address, and, in some
embodiments, may also contain a start of data pattern. The clock
acquisition section contains timing and AGC fields with no data in
between them so that amplitude modulation can be removed and the
clock can be acquired. The level calibration pattern contains long
signals at each level to calibrate the system. The alignment
sequence is a pseudo random sequence that has a response such that
when the alignment sequence is correlated with itself, the
correlation has a sharp peak; that is the correlation has a
substantially high value at only a location that indicates precise
alignment. The equalizer training sequence enables the adjustment
of the equalizer to the particular disc and player combination
being read. The block address enables the system to uniquely
identify the ECC block on the disc. In some embodiments, a start of
data pattern sequence identifies the start of the data pattern. In
other embodiments, the start of the data pattern is determined by
an offset from the alignment block.
[0059] The ECC block includes modulation encoded marks and physical
format marks. The modulation encoded marks are encoded by the
modulation so that their form is altered by the modulation code and
the physical format marks are periodically inserted in the data
stream after modulation coding so that their form is not altered by
the modulation coding. The ECC block (which is shown in FIG. 5C)
contains an encoded address section, periodic ECC-data synch
fields, periodic timing fields, periodic AGC fields, periodic DC
control fields, encoded data marks, encoded ECC marks, and encoded
trellis clean-up marks. The physical format ECC block marks include
periodic ECC-data synch fields, periodic timing fields, periodic
AGC fields, and periodic DC control fields. The physical format
marks plus the modulation encoded marks comprise the ECC block.
[0060] The data marks (g) are interspersed between the other format
marks or fields (groups of marks). The address section (a)
duplicates the address of the ECC block and also contains an error
detection code. The ECC-data synch (b) fields provide location
information within the ECC block to aid data recovery. As described
above, the timing fields (c) are used to recover the clock, and the
AGC (d) fields are used to remove the low-frequency drift in the
envelope. The DC control (e) fields are used to control the DC
content of the signal. The ECC (f) fields contain the error
correction bytes that enable locating and correcting errors in the
data stream. Finally, the encoded trellis clean-up marks (h) are
used to return the trellis coded marks to a known state at periodic
points in the ECC block.
[0061] FIG. 5D illustrates a postamble sequence. The sequence
includes timing fields (c), AGC fields (d), filler data marks (g),
and DC control fields (e). DC control fields are used to reduce the
output DC level of the data block structure. In some embodiments,
the postamble may also contain encoded trellis clean-up marks (h)
to leave the trellis in a known state. FIG. 6 is a block diagram
illustrating a system for writing multilevel marks to a disc. Data
from a data buffer 600 is input to an ECC encoding block 601 where
ECC check bytes are added to the data stream. This data is then fed
to a multilevel modulation block 602. Multilevel modulation block
602 includes a convolution coding block 604 and a mark block 605
that adds the ECC-data synch fields, the DC control fields, and run
length limited (RLL) constraints, if such constraints are
implemented. Timing and automatic gain control fields such as are
described above are added in a block 606. The data stream is then
sent to a precompensator 608 and the output of the precompensator
is sent to a multilevel implementer 610 that translates the data
stream into a bit stream in accordance with the laser write
strategy selected for the disc being recorded.
[0062] In this embodiment, the ECC encoded data is convolutionally
encoded. Convolutional encoding adds correlations between data
marks. Both these correlations and correlations due to the optical
channel's inherent intersymbol interference can enable more
accurate decoding of the data sequence by a maximum likelihood
detector, such as a Viterbi detector. In one embodiment, in order
to achieve a data rate of 3 bits/mark with M=12 levels, the
modulation is 2-dimensional such that 6 bits plus 1 parity bit are
encoded into a symbol consisting of two adjacent marks. Since 7
bits require 128 symbols, 16 of the possible 12.times.12=144
symbols are not be used. The 128-cross modulation constellation is
given below. The symbol assignment is found by taking first the row
and then the column number for the value to be encoded. For
example, the 7-bit value 64 would be encoded as two marks of levels
4 and 7. 999 indicates an invalid symbol.
1 11 999 999 20 97 16 101 100 17 96 21 999 999 10 999 999 107 110
31 26 27 30 111 106 999 999 9 12 41 8 45 92 89 88 93 44 9 40 13 8
35 38 87 2 83 6 7 82 3 86 39 34 7 48 53 68 113 64 117 116 65 112 69
52 49 6 127 58 123 62 79 74 75 78 63 122 59 126 5 124 57 120 61 76
73 72 77 60 121 56 125 4 51 54 71 114 67 118 119 66 115 70 55 50 3
32 37 84 1 80 5 4 81 0 85 36 33 2 15 42 11 46 95 90 91 94 47 10 43
14 1 999 999 104 109 28 25 24 29 108 105 999 999 0 999 999 23 98 19
102 103 18 99 22 999 999 0 1 2 3 4 5 6 7 8 9 10 11
[0063] The convolutional encoding is chosen from Table III of
"Trellis-Coded Modulation with Redundant Signal Sets, Part 1,"
Gottfried Ungerboeck, IEEE Communications Magazine, February 1987.
The choice is made to balance complexity against coding gain.
[0064] Mark Block 605 adds the ECC-data synch patterns which are
composed of symbols chosen from the 16 symbols not found in the
modulation constellation. One embodiment is given below.
2 Sync0: 0 11 11 0 0 11 11 0 0 11 Sync1: 11 0 0 11 11 0 0 11 11 0
Sync2: 1 10 10 1 1 10 10 1 1 10 Sync3: 10 1 1 10 10 1 1 10 10 1
Sync4: 10 10 1 1 10 1 11 11 0 0 Sync5: 0 0 11 11 1 10 1 1 10 10
Sync6: 11 11 0 0 10 1 10 10 1 1 Sync7: 1 1 10 10 1 10 0 0 11 11
Sync8: 0 1 11 10 1 0 1 11 10 10 Sync9: 11 0 11 0 11 0 11 0 11 0
Sync10: 11 10 0 0 1 10 10 1 11 1 Sync11: 10 1 10 1 10 1 10 1 10 1
Sync12: 1 10 11 11 0 0 1 10 11 0
[0065] Next the DC control fields are added. The DC content of the
data signal can be measured using a running digital sum (RDS). In a
12-level system, the each level is assigned a DC content value. For
example, level 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 are assigned DC
content values -11, -9, -7, -5, -3, -1, 1, 3, 5, 7, 9, 11
respectively. RDS is the running sum of the DC content values as
the data levels are read. By maintaining the RDS near zero, the
average data signal level is near center of the amplitude range of
the signal. In one embodiment, the DC control fields signify the
inversion or not of the data that is found after one DC control
field. For the 12-level modulation code above, inversion would mean
that the data mark level k would become data mark level 11-k. In
other words, a level 11 would become 0 and a level 3 would become
8. The inversion helps to control the DC content of the data
stream. RLL marks can end runs of data at the same level by
inserting a mark of a different level at particular locations in
the data stream. In one embodiment, the timing, AGC, and DC control
fields are frequent enough to act as RLL marks. In other
embodiments, separate RLL marks may need to be added.
[0066] The marks are then sent on to the precompensator which
adjusts the levels desired taking into account the neighboring
marks to remove the effects of intersymbol interference or else
cause a certain intersymbol interference target to be realized.
[0067] If the intersymbol interference is precompensated to a
specific target intersymbol interference, the intersymbol
interference can be used by the Viterbi detector to better decode
the data. The intersymbol interference adds correlations into the
data stream so that maximum likelihood detection, as is done when
using a Viterbi detector, can better interpret the data signal.
Correlations can also be introduced into the data stream explicitly
using a convolutional code. In various embodiments, correlations
are introduced using several different methods or combinations of
such methods. For example, correlations may be introduced by the
optical system, can be shaped by using precompensation, can be
introduced using a convolutional code, and can be introduced by
both ISI and convolutional encoding.
[0068] In addition, in certain embodiments, write calibration is
included with the multilevel implementer. Write calibration
compensates for changes in the writing of marks on the disc due to
age or wear as well as variations in disc characteristics. The
write calibration procedure has 3 iterative steps: 1) write a known
pattern to the disc, 2) read the pattern, 3) adjust the write
strategy to ensure that the pattern written causes the pattern that
is read to be the desired read pattern. In one embodiment, the read
pattern is compared to a linear filtered version of the written
pattern. In this embodiment, the adjustments are made so that the
non-linear effects of the system are removed or compensated for by
calibration. In other embodiments, both the linear and the
non-linear effects of the read and write system can be calibrated
or compensated for using this method.
[0069] This procedure allows the player and disc combination to
adjust for changes that may affect the writing and reading of data
onto the disc. Factors that might be calibrated out using this
procedure include laser age, laser temperature, dust on the lens or
disc, and variation of the disc materials.
[0070] Finally, the adjusted levels are sent to the multi-level
implementer which translates the levels determined using the
precompensator and the write calibrator to a series of pulses of
specific laser powers of specific durations at specific times.
[0071] A system for writing data to a multilevel disc and reading
data from the multilevel disc has been disclosed. The read system
compensates for noise introduced in the read signal by using an
adaptive FSE. In addition, precompensation is performed before a
signal is written to the disk. Special fields are written to the
disc to facilitate clock recovery and automatic gain compensation.
Other fields control the DC bias of the signal read from the
disk.
[0072] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the appended claims. It should be noted that
there are many alternative ways of implementing both the process
and apparatus of the present invention. Accordingly, the present
embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details
given herein, but may be modified within the scope and equivalents
of the appended claims.
* * * * *